The present invention relates to attaching a die or other material to another surface, and in particular, relates to using a time-based technique for controlling such attachment.
The semiconductor industry uses a variety of techniques to attach semiconductor die to bond pads, substrates, or like semiconductor materials during manufacturing. The equipment used to facilitate such attachment is commonly referred to as a die bonder. A die bonder typically operates to pick up a semiconductor die and attach it to another semiconductor structure using an epoxy or like adhesive. Die bonders are well known in the art, and are available from a variety of equipment vendors. A continuing problem with existing die bonders is the inability to accurately control the thickness of the epoxy layer between the semiconductor die and the structure to which it is attached. The resulting thickness of the epoxy after attachment is commonly referred to as the bond line thickness (BLT).
The die bonder generally incorporates a placement tool that has a head, which picks up a semiconductor die, positions it over the structure to which the semiconductor die is to be mounted, and lowers the semiconductor die onto the structure for attachment. Notably, the epoxy is previously applied to the surface upon which the semiconductor die is to be mounted. Existing die bonders struggle to provide BLT accuracies within 25 to 50 microns with relatively large semiconductor die. For relatively small semiconductor die, such as those having a top surface area of less than 2 mm square, and in particular, for semiconductor die less than 1 mm by 1 mm, these machines are incapable of providing reliable accuracy during attachment. Further, smaller semiconductor die typically require an actual BLT, which is often less than the variance for existing die bonders. For example, most die bonders provide BLT accuracies within plus or minus 20 microns, wherein smaller semiconductor die may require an actual BLT of 12 microns plus or minus 2 or 3 microns. As such, existing configurations of die bonders inject intolerable variations in construction parameters for the semiconductor.
Existing die bonders typically use one of two techniques for attaching the semiconductor die to a desired mounting surface. The first technique uses force to determine when to stop lowering the semiconductor die toward the mounting structure. For smaller semiconductor die and smaller BLT requirements, existing systems are not sensitive enough to detect any force prior to the semiconductor die being forced against the mounting structure, and thus, forcing the epoxy out from between the semiconductor die and the mounting structure. As such, maintaining a relatively small BLT is virtually impossible, because the semiconductor die is forced against the mounting surface. The second technique relies on moving the semiconductor die to a set height above the mounting structure. For smaller semiconductor die and smaller BLT requirements, using a set height is unacceptable because mounting substrates often vary many times more than the actual bond line thickness.
Accordingly, there is a need for a semiconductor die attachment technique capable of providing accuracy far surpassing that of existing systems. There is a further need to provide this technique for smaller semiconductor die having smaller BLT requirements. Preferably, the technique would provide BLTs having an accuracy of plus or minus a few microns.
The present invention controls attachment of a first semiconductor material, such as a semiconductor die, to a second semiconductor material, such as a bond pad, substrate, or the like. A placement tool is used to pick up the first semiconductor material and move it to a defined position above the top surface of the second semiconductor material. The second semiconductor material will have an adhesive, such as epoxy, applied to its top surface From the defined position above the second semiconductor material, the placement tool is allowed to fall for an amount of time previously determined to result in an adhesive layer of a defined thickness, within precise tolerances. The adhesive thickness is often referred to as bond line thickness (BLT) when bonding a semiconductor die to a bond pad, substrate, or the like. After falling for the predefined period of time, the placement tool is stopped and removed, wherein the first semiconductor material remains adhered above the top surface of the second semiconductor material at a defined height, which correlates to an adhesive layer having a predefined thickness.
Preferably, a die bonder is used to control placement of the placement tool. The die bonder may use a vacuum-based control system to control vertical movement of the placement tool. As such, when in the first position above the second semiconductor material, the vacuum is shut off and the placement tool falls toward the second semiconductor material as the vacuum decays. After the predetermined period of time has elapsed, the descent of the placement tool is stopped, wherein the placement tool may be removed to pick up another first semiconductor material for placement. The defined period of time for descent of the tool and first semiconductor material includes a first period wherein the placement tool and first semiconductor material descend through air prior to impacting the adhesive on the second semiconductor material. The placement tool and first semiconductor material will continue to fall into the adhesive, wherein the adhesive is spread and compressed evenly between the bottom surface of the first semiconductor material and the top surface of the second semiconductor material. As noted, the period of time for descent is sufficient to result in an adhesive thickness of a predefined dimension within precise tolerances. For best results, the viscosity, amount, and shape of the adhesive initially placed on the top surface of the second semiconductor material remains constant for consecutive operations.
The present invention may be implemented in the form of a process or method on various types of bonding systems. Further, the concepts of the invention may be embodied in software used to carry out control of these systems.
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.